1. Field of the Invention
The present invention is directed generally to an improved process for the steam reforming of hydrocarbon gas feeds, and specifically to an improved low severity steam reforming process.
2. Description of the Prior Art
Generally, the manufacture of ammonia consists of preparing an ammonia synthesis gas from a nitrogen source, usually air, and from a hydrogen source, which is conventionally either coal, petroleum fractions, or natural gases. For example, in the preparation of ammonia synthesis gas from a light hydrocarbon feedstock, which may range from natural gas to naphtha, the hydrocarbon feedstock gas is first purified by removing gaseous contaminants, such as sulfur (which would poison the downstream catalysts) from the feedstock by the catalytic hydrogenation of the sulfur compounds to hydrogen sulfide and adsorption of the hydrogen sulfide over a zinc oxide adsorption medium. Subsequent steam reforming of the contaminant-free gas provides the major portion of the hydrogen required for ammonia synthesis from the hydrocarbons in the gas. Reforming is accomplished by a two-stage process in which a mixture of steam and the purified feed gas are first reformed over catalyst in a primary reformer, followed by treatment of the partially reformed gas in a secondary reformer to which air is introduced, in order to provide the required amount of N.sub.2 for ammonia synthesis. A reformed gas is produced in the secondary reformer having a greater amount of hydrogen and a lesser amount of hydrocarbons. The reaction processes occurring in the reforming of the feedstock gas begin with the breakdown of hydrocarbons to methane, carbon dioxide and carbon monoxide: EQU H.sub.2 O+C.sub.n H.sub.(2n+2).fwdarw.CH.sub.4 +CO+CO.sub.2 +H.sub.2
and end with the reforming of these products by the desired endothermic methane reforming reaction: EQU CH.sub.4 +H.sub.2 O.fwdarw.CO+3H.sub.2
and by accompanying exothermic reactions: EQU 2CH.sub.4 +7/2 O.sub.2 .fwdarw.CO.sub.2 +CO+4H.sub.2 O EQU CO+H.sub.2 O.fwdarw.CO.sub.2 +H.sub.2 EQU 2H.sub.2 +O.sub.2 .fwdarw.2H.sub.2 O EQU CO+1/2 O.sub.2 .fwdarw.CO.sub.2
The carbon monoxide in the reformed gas is converted to carbon dioxide and additional hydrogen in one or more shift conversion vessels, and the carbon dioxide is removed by scrubbing. Further treatment of the raw synthesis gas by methanation may be used to remove additional carbon dioxide and carbon monoxide from the hydrogen-rich gas, resulting subsequently in an ammonia synthesis gas containing approximately three parts of hydrogen and one part of nitrogen, that is, the 3:1 stoichiometric ratio of hydrogen to nitrogen in ammonia, plus small amounts of inerts such as methane, argon and helium. The ammonia synthesis gas is then converted to ammonia by passing the gas over a catalytic surface based upon metallic iron (conventionally magnetite) which has been promoted with other metallic oxides, and allowing the ammonia to be synthesized according to the following exothermic reaction: EQU N.sub.2 +3H.sub.2 .fwdarw.2NH.sub.3
The effluent from the ammonia reactor, which contains ammonia, unconverted H.sub.2 and N.sub.2, and gases which are essentially inerts in the ammonia reaction (principally, methane and argon), is then treated for ammonia recovery, and to form a recycle stream containing H.sub.2 and N.sub.2 which can be returned to the ammonia reactor along with fresh ammonia synthesis gas.
In a conventional steam reforming ammonia process, it is desirable to minimize the amount of unconverted hydrocarbons (methane slippage) leaving the reformers. Methane will concentrate in the feed to the ammonia reactors because H.sub.2 and N.sub.2 react and are removed as ammonia product, but the major portion of the inerts such as methane is recycled along with the H.sub.2 and N.sub.2 and remains in the reactor/recycle loop. If inerts buildup were left unchecked, the partial pressure of the reactants (hydrogen and nitrogen) would be reduced to the point where the reaction rate would be uneconomically slow. To prevent this excessive buildup, an inerts purge is generally taken, conventionally going to fuel. Unfortunately, valuable hydrogen (and nitrogen) is lost in the purge, which typically contains only 10-20% inerts. Minimizing reformer methane slippage minimizes this loss, at the expense of higher reformer furnace fuel and investment requirements. Balances among these factors lead to conventional designs having 7-12 dry mole % methane in the primary reformer effluent.
Recent years have seen the development of various schemes for recovering most of the hydrogen from the inerts purge stream. These purge recovery units have been based on cryogenic fractionation, pressure swing adsorption or membrane diffusion. What they all have in common is that they produce two streams: one enriched in hydrogen for recycle back to the ammonia reactor, and one enriched in inerts going to fuel. An alternative approach to minimizing purge hydrogen loss is the Kellogg purge converter scheme, which employs a second ammonia synthesis reactor to recover part of the purge gas hydrogen and nitrogen as ammonia product. Regardless of which approach is used, a purge gas hydrogen recovery unit saves energy by increasing the conversion of feed to product by minimizing the amount of valuable hydrogen downgraded to fuel. In a high energy cost environment, the investment for a purge recovery unit has often been justified on this basis, without significant change to the operating conditions in the reforming section of the plant.
U.S. Pat. No. 3,081,268 (1963) employs an externally fired primary reformer with high excess steam (steam to feed gas carbon mole ratio of 4 to 8) to achieve an exit gas having a temperature of from about 1350.degree. F. to 1650.degree. F. and a pressure of from about 50-200 psi to achieve conversion of 65 to 85% of the feed hydrocarbons to H.sub.2 and carbon oxides. This hydrocarbon conversion level is increased further to 95%-99% overall, in a secondary reformer. The secondary reformer effluent is treated in a shift converter for CO removal, cooled and purified to form the synthesis gas to the ammonia reactor.
U.S. Pat. No. 3,442,613 (1969) to C. F. Braun & Company disclose a process wherein excess methane and argon present in the methanator effluent, are removed ahead of the ammonia synthesis reaction zone by cryogenic techniques to form a high purity synthesis gas and to allow minimization of ammonia synthesis loop purge requirements. Related to the Braun patent are B. J. Grotz, Hydrocarbon Processing, vol. 46, no. 4, pp. 197-202 (April 1967) and B. J. Grotz, Nitrogen, vol. 100, pp. 71-75 (1976), and U.K. Patent Nos. 1,156,002 and 1,156,003.
In U.S. Pat. No. 3,441,393 (1969) a Pullman process is disclosed wherein the reforming, shift conversion and methanation steps are accomplished such as to form an ammonia synthesis gas and an ammonia reactor effluent gas dilute in NH.sub.3 (about 9.7% NH.sub.3). A purge gas stream must be taken to avoid inerts build-up, and the inerts level in the recycle stream to the reactor is such that the combined (recycle and fresh syn gas) feed to the reactor has from about 5 to 20 mol. % inerts.
U.S. Pat. No. 3,947,551 (1976) to the Benfield Corporation relates to a process in which the primary and secondary reforming conditions are such that a low methane concentration (about 0.3% CH.sub.4) is present in the secondary reformer effluent. Following shift conversion, CO.sub.2 removal and methanation, the ammonia synthesis gas is combined with a recycle gas and passed to ammonia synthesis. Published U.K. Patent Application No. 2,017,071A (1979) to Monsanto forms NH.sub.3 from an ammonia synthesis gas containing from 2-15 vol. % CH.sub.4, and H.sub.2 :N.sub.2 mole ratios of from 2:1 to 4:1, to form a reactor product gas containing from 10-25% NH.sub.3.
U.S. Pat. No. 4,298,588 (1981) to ICI relates to a process wherein primary reforming is accomplished with total steam to carbon ratio of 2.5 to 3.5:1 to form an exit gas (750.degree.-850.degree. C., 30-120 bar) containing at least 10% and not more than 30% CH.sub.4, followed by secondary reforming with excess air (above stoichiometric) to provide an effluent gas (950.degree.-1050.degree. C., about 30-120 bar) having from 0.2-10% CH.sub.4 and a H.sub.2 :N.sub.2 mole ratio of from 2.0 to 2.9:1. After shift conversion, CO.sub.2 removal and methanation, the resulting fresh synthesis gas (said to contain usually under 1% v/v of methane) is combined with a H.sub.2 -rich recycle gas stream (at a ratio of recycled gas to fresh gas of 4 to 6) and passed to an ammonia reactor to give an ammonia reactor effluent gas containing 8 to 12 v/v % NH.sub.3. After removal of the ammonia product, the remaining gas is partially recycled to the reactor and partially sent to a purge recovery unit for removal of inerts and the excess N.sub.2 (above stoichiometric) introduced with the fresh synthesis gas. However, since H.sub.2 recovery in conventional purge recovery units is not complete, this process results in a high H.sub.2 loss rate due to the need for a high flow rate of gases to be treated in the purge recovery unit.
U.S. Pat. No. 4,213,954 (1980) to ICI employs reforming conditions and gas recycles similar to U.S. Pat. No. 4,298,588.
Published European Application No. 49,967 (1982) to ICI employs an adiabatic primary reforming step at lower primary reformer outlet temperatures (&lt;750.degree. C., e.g., 550.degree.-650.degree. C.) than those in the above discussed U.S. Pat. No. 4,298,588. The primary reformer effluent, containing 25 to 35% CH.sub.4, is subjected to secondary reforming with excess air at secondary reformer outlet temperatures of &lt;900.degree. C., to form an outlet gas containing from 1.5-3.0 mol. % CH.sub.4 (dry basis) and a low H.sub.2 :N.sub.2 ratio (1.0-2.5:1), and produce an ammonia reactor effluent gas (after shift cohversion, CO.sub.2 removal and methanation) having an ammonia content of about 14 mol. % which is passed to ammonia recovery and thence partially to a purge gas recovery unit to form a H.sub.2 -rich recycle stream. Again, high H.sub.2 losses result due to the need to treat a large volume of gases in the purge recovery unit.
Illustrative purge recovery units are discussed in U.K. Patent Nos. 1,057,020; 1,460,681 and 1,274,504; U.K. Patent Application No. 2,030,473A; Russian Pat. No. 486,667 (1973); R. Banks, Chem. Eng., pp. 90-92 (Oct. 10, 1977); A. Haslam, et al., Hydrocarbon Processing, pp. 103-106 (January 1976); K. S. Chari, Chem. Age India, pp. 283-285 (April 1978), T. Matsuoka, Chem. Age India, vol. 30, no. 2, pp. 119-128 (February 1979); and R. L. Shaner, Chem Eng. Prog., pp. 47-52 (May 1978).